1. Field of the Invention
The present invention relates to an illumination optical system using a reflecting light valve such as a digital micromirror device (referred to as xe2x80x9cDMDxe2x80x9d hereinafter) or a reflecting liquid crystal display. It also relates to a projection display (e.g., projector) including the illumination optical system.
2. Description of the Background Art
FIG. 14 is a schematic diagram of an illumination optical system applied to a conventional projection display disclosed in Japanese Patent Application Laid-open No. 2000-241755. In FIG. 14, reference numeral 31 designates a light source and reference numeral 32 designates a parabolic mirror that reflects light from the light source 31 and makes the reflected light generally parallel to an optical axis of illumination. Reference numeral 33 designates a first lens array, reference numeral 34 designates a convex lens constituting the first lens array 33, reference numeral 35 designates a second lens array, and reference numerals 36 and 37 designate convex lenses constituting the second lens array 35. Further, reference numeral 38 designates a collective lens and reference numeral 39 designates a to-be-illuminated surface.
Light emitted from the light source 31 and then reflected by the parabolic mirror 32 is generally made parallel; however, in the open end of the parabolic mirror 32, the light intensity distribution in the periphery is higher than that in the center because of the influences of optical properties of the light source 31 such as light emission and light distribution. The illumination optical system of FIG. 14 has the functions of reducing such a deviation of the illumination light distribution and illuminating the central portion of the to-be-illuminated surface more brightly than the peripheral portion. Now, the operation of the illumination optical system in FIG. 14 will be briefly described.
The first lens array 33 is formed of an array of a plurality of convex lenses 34 having the same focal length. The second lens array 35, on the other hand, is formed of an array of alternate convex lenses 36 and 37 having two different focal lengths. The focal length of the convex lenses 36 is determined so as to adjust, in combination with the condenser lens 38, the focus of their corresponding convex lenses 34 in the first lens array 33 on the to-be-illuminated surface 39. From this, light passing through half of the convex lenses 34 in the first lens array 33 is superimposed on the to-be-illuminated surface 39, and even if the light intensity distribution in the open end of the parabolic mirror 32 is not uniform, the to-be-illuminated surface 39 can be provided with a uniform illuminance distribution.
The focal length of the convex lenses 37, on the other hand, is determined to be greater than that of the convex lenses 36; therefore, even in combination with the condenser lens 38, the focus of their corresponding convex lenses 34 in the first lens array 33 cannot be adjusted on the to-be-illuminated surface 39. That is, light passing through the convex lenses 37 has a convexly curved illuminance distribution on the to-be-illuminated surface 39, with the central portion being higher in illuminance than the peripheral portion.
As above described, the illuminance distribution on the to-be-illuminated surface 39 is formed by a combination of (i) the uniform illuminance distribution of the light passing through the convex lenses 36 and (ii) the convexly curved illuminance distribution of the light passing through the convex lenses 37. That is, the illuminance distribution on the to-be-illuminated surface 39 gently changes within an effective illuminated area on the to-be-illuminated surface 39, with the central portion being higher in illuminance than the peripheral portion. This achieves a projection display capable of making improvement on uniform illumination that is not suitable for TV image display, for example.
In the above conventional projection display illustrated in FIG. 14, however, the light passing through the convex lenses 37, which account for one half of the total lenses in the second lens array 35, is not focused on the to-be-illuminated surface 39. Accordingly, a light source image to be formed on the pupil of a projection lens (not shown) is out of focus as well. Considering efficiency of light utilization, the light source image on the pupil should be in sharp focus. From this, the conventional projection display in FIG. 14 has a problem that the positive provision of the convex lenses 37 causes defocusing as above described, thereby resulting in unavoidable energy loss.
A first aspect of the present invention is directed to an illumination optical system comprising: a light source; a light mixing element mixing light emitted from the light source; a reflecting light valve having a plurality of pixels; a transmission optical system located between the light mixing element and the reflecting light valve and bringing a light exit surface of the light mixing element into optical conjugation with a to-be-illuminated surface of the reflecting light valve; and a field lens located between the transmission optical system and the reflecting light valve, wherein a focal length of the field lens is determined so that an illuminance distribution which varies according to the application of the illumination optical system can be achieved on the to-be-illuminated surface.
According to a second aspect of the present invention, in the illumination optical system of the first aspect, the focal length of the field lens is determined so that an illumination margin takes on a value in the neighborhood of 1.0, and a diagonal dimension of a to-be-illuminated area on the to-be-illuminated surface is equivalent to the product of a diagonal dimension of the to-be-illuminated surface and the illumination margin.
According to a third aspect of the present invention, in the illumination optical system of the first aspect, the focal length of the field lens is determined by obtaining at least one of a radius of curvature of the field lens and the type of a glass material forming the field lens on the basis of the illuminance distribution which varies according to the application of the illumination optical system.
According to a fourth aspect of the present invention, in the illumination optical system of the first aspect, the light exit surface of the light mixing element and the to-be-illuminated surface of the reflecting light valve are generally similar in shape.
According to a fifth aspect of the present invention, in the illumination optical system of the first aspect, the plurality of pixels in the reflecting light valve each are configured of a micromirror having a variable angle of inclination.
According to a sixth aspect of the present invention, in the illumination optical system of the first aspect, the light mixing element is formed in the shape of a hollow rod, using its inner surface as a reflecting surface.
According to a seventh aspect of the present invention, the illumination optical system of the first aspect further comprises: a rotary color filter located either in front of or behind the light mixing element to pass the light emitted from the light source.
According to an eighth aspect of the present invention, in the illumination optical system of the first aspect, the field lens is a planoconvex lens having a plane surface on the side of the to-be-illuminated surface of the reflecting light valve.
According to a ninth aspect of the present invention, in the illumination optical system of the first aspect, the field lens has at least one aspherical surface.
According to a tenth aspect of the present invention, in the illumination optical system of the first aspect, the field lens is a Fresnel lens.
An eleventh aspect of the present invention is directed to a projection display comprising: the illumination optical system of the first aspect; a screen; and a projection lens system located between the reflecting light valve and the screen.
The first, second, and eleventh aspects of the present invention can achieve the effect of, by simply changing the focal length of the field lens located in front of the reflecting light valve, easily achieving various illuminance distributions depending on applications of the illumination optical system on the reflecting light valve, without reducing the efficiency of light utilization.
The third aspect of the present invention can achieve the effect of, by simply replacing the field lens, achieving different illuminance distributions on the reflecting light valve without giving any change to the optical elements other than the field lens and to the structural members holding the optical elements.
The fourth aspect of the present invention can achieve the effect of allowing easy design of the light mixing element and minimized optical losses.
The fifth aspect of the present invention can achieve the effect of providing a low-cost, light-efficient device.
The sixth aspect of the present invention can achieve the effect of simplifying the ways of cooling and holding the light mixing element.
The seventh aspect of the present invention can achieve the effect of providing a compact, light-efficient device.
The eighth aspect of the present invention can achieve the effect of facilitating the settings of the relative positions of the field lens and the reflecting light valve.
The ninth and tenth aspects of the present invention can achieve the effect of achieving high image performance while correcting high-order off-axis aberration such as distortion, coma, and astigmatism.
The present invention has been made in view of the aforementioned conventional problems and an object thereof is to achieve, on a reflecting light valve, various illuminance distributions depending on applications of the illumination optical system or projection display, while improving the efficiency of light utilization without energy loss. More specifically, the principal object of the present invention is to achieve (i) a uniform illuminance distribution on a to-be-illuminated surface of the reflecting light valve and conversely (ii) a non-uniform illuminance distribution on the to-be-illuminated surface of the reflecting light valve.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.